A wide variety of coatings have been used to coat the surfaces of two-piece food and beverage cans. These cans are generally coated using “coil coating” operations, i.e., a planar sheet of a suitable metal substrate (e.g., steel or aluminum metal) is coated with a suitable composition and cured and then the coated substrate is formed into the can end or body. The coating should be capable of high-speed application to the substrate and provide the necessary properties when cured to perform in this demanding end use. For example, the coating should be safe for food contact; have excellent adhesion to the substrate; be capable of being drawn during the forming step; when used as an end coating, provide clean edges when the end is opened; resist staining and other coating defects such as “popping,” “blushing” and/or “blistering;” and resist degradation over long periods of time, even when exposed to harsh environments. Previous coatings have suffered from one or more deficiencies.
Various coatings have been used as interior protective can coatings, including epoxy-based coatings and polyvinyl-chloride-based coatings. Each of these coating types, however, has potential shortcomings. For example, the recycling of materials containing polyvinyl chloride or related halide-containing vinyl polymers can be problematic. There is also a desire by some to reduce or eliminate certain epoxy compounds commonly used to formulate food-contact epoxy coatings.
To address the aforementioned shortcomings, the packaging coatings industry has sought coatings based on alternative binder systems such as polyester resin systems. It has been problematic, however, to formulate polyester-based coatings that exhibit the required balance of coating characteristics (e.g., flexibility, adhesion, corrosion resistance, stability, resistance to crazing, etc.). For example, there has typically been a tradeoff between corrosion resistance and fabrication properties for such coatings. Polyester-based coatings suitable for food-contact that have exhibited both good fabrication properties and an absence of crazing having tended to be too soft and exhibit unsuitable corrosion resistance. Conversely, polyester-based coatings suitable for food contact that have exhibited good corrosion resistance have typically exhibited poor flexibility and unsuitable crazing when fabricated.
What is needed in the marketplace is an improved binder system for use in coatings such as, for example, packaging coatings. Such packages, compositions and methods for preparing the same are disclosed and claimed herein.